We reported previously that several mutations that abolish either of the two mechanisms of replication repair are general mutator mutations that sometimes promote templated mutations particularly strongly. Thus, defects in replication repair somehow promote template switching to an inappropriate template or prevent the return of the primer strand to its cognate template. We are now testing whether DNA damage can promote templated mutagenesis by introducing polymerase-blocking lesions into phage T4 using ultraviolet irradiation.